Method for detecting the spatial position of a tracking mirror and a mirror arrangement for carrying out said method

ABSTRACT

In a laser tracking system equipped for interferometric distance measurement there are provided at least two retroreflectors ( 3.1, 3.2, 3.3 ) which are connected to the target tracking mirror ( 1 ) in a manner such that their position changes when the spatial orientation of the target tracking mirror ( 1 ) is changed. Secondary measurement beams ( 4.1, 4.2, 4.3 ) deflected out of the primary measuring beam ( 4 ) of the laser tracking system are directed onto the retroreflectors ( 3.1, 3.2, 3.3. ). Path length changes in the beam path of the secondary measurement beams ( 4.1, 4.2, 4.3 ) are interferometrically measured and the readings are used for computing the spatial orientation of the target tracking mirror ( 1 ).

The invention relates to a method according to the preamble of the firstindependent patent claim. The method serves for determining the spatialorientation of a target tracking mirror in a laser tracking system. Thesystem further relates to a mirror arrangement according to thecorresponding independent patent claim, said mirror arrangement servingfor carrying out the method.

The target tracking mirror is an essential component of laser trackingsystems as they have been known for many years now in industry and areused for precise coordinate measurements on large subjects. Lasertracking systems permit the tracking of a moving target retroreflectorwith a tracking measurement beam, wherein by way of suitablemeasurements of the direction of the measurement beam and by way ofinterferometric measurements of the distance to the targetretroreflector the coordinates (e.g. polar coordinates) of the targetretroreflector are determined.

Directing the tracking measurement beam in a manner such that it alwaysimpinges the moving retroreflector is effected by suitably adjusting thetarget tracking mirror which usually is rotatable about two axesstanding perpendicular to one another. The tracking measurement beam isreflected back by the retroreflector to the target tracking mirror andfrom this is deflected to the interferometer receiver. Theinterferometer receiver determines the distance of the targetretroreflector to a defined zero position of the laser interferometer.The spatial orientation of the target tracking mirror is determined bymeasurement and from the readings the direction of the measurement beamis computed.

For rotating the target tracking mirror about the axes usuallyservomotors assembled on the axes are provided. For determining theorientation of the target tracking mirror, with respect to a pregivenzero orientation usually each of the axes is equipped with an angleencoder. A typical laser tracking system equipped in such a manner witha target tracking mirror is for example described in the publicationU.S. Pat. No. 4,714,339. This system comprises a target tracking mirrorarranged in a cardanic suspension and therefore being rotatable abouttwo axes standing perpendicular to one another.

In Applied Optics, Volume 2, No. 7, July 1963, page 762 ff. the use of aMichelson interferometer is described as an alternative to angleencoders for angle measurement in order to determine the rotationalmovement in a gamma beam spectrometer. The Michelson interferometer is atwo-armed interferometer with two equally long arms, one for thereference beam path and the other for the measurement beam path. In thedescribed application the measurement beam is directed in anunchangeable manner towards a die-corner prism, the prism being arrangedon a part which is rotatable about an axis. The measurement beam isdeflected by the prism to a stationary mirror and by the mirror to thesame path back to the interferometer. Between the interferometer andprism, the measurement beam runs parallel to a tangent on the circulararc described by the prism on rotating about the axis. The referencebeam is reflected by a stationary mirror to the interferometer. Withrotation of the prism about the rotational axis the measurement beampath is lengthened or shortened and this is interferometricallydetected. From the change of the path length of the measurement beampath when moving the prism from a predetermined zero position to amomentary position the rotational angle of the prism with respect tothis zero position is calculated.

The described arrangement of prism and mirror renders it necessary forthe measurement beam to eccentrically impinge the same prism side inevery possible rotational position of the prism. This means that theopening of the prism and thus the prism itself must be relatively large.Therefore, the prism is relatively heavy so that it may influences themoment of inertia of the rotating part in a relevant manner.

It is the object of the invention to provide a method for determiningthe spatial orientation of a target tracking mirror in a laser trackingsystem, wherein the method according to the invention with respect toknown methods for determining the spatial orientation of such a targettracking mirror is to permit a higher accuracy and wherein the method isto permit a very compact mirror arrangement in which the parts to bemoved with the mirror are to be as light as possible so that as small aspossible inertia stands in the way of a movement of the mirror. It isfurthermore the object of the invention to provide a mirror arrangementfor carrying out the method according to the invention. This arrangementis to be compact and possibly able to be integrated into known lasertracking systems in a modular manner.

This object is achieved by the method and the device as defined in theindependent patent claims. Further advantageous embodiments of theinvention are the subject-matter of the dependent claims.

The method according to the invention is based on the idea ofdetermining the spatial orientation of the target tracking mirror of thelaser tracking system not with the help of angle encoders arranged onrotational axes but with interferometric methods. For this purpose atleast two retroreflectors are connected to the target tracking mirrorand are therefore moving together with the target tracking mirror. Foreach one of the reflectors a secondary interferometric measurementsystem is provided, wherein the measurement and reference beams of thesecondary measurement systems are branched out of the primary beam pathof the laser tracking system. In each secondary measurement system asecondary measurement beam with an unchangeable direction is directedonto one of the retroreflectors moving with the target tracking mirrorand the change of the length of the beam path of the secondarymeasurement beam is interferometrically determined when the mirror ismoved. Retroreflectors and secondary measurement systems are arrangedsuch that mirror movements effect path length changes for the secondarymeasurement beams and that the measurement data gained from such pathlength changes which characterize distances of a momentary reflectorposition from a predetermined zero position in the direction of themeasurement beam, allow unambiguous computation of the mirrororientation.

The movements of a target tracking mirror are usually rotation movementsin which the retroreflectors allocated to the secondary measurementsystems move on a circular arc also. Each secondary measurement beam isdirected tangentially onto such a circular arc, advantageously in amanner such that it impinges the retroreflector essentially centrallywhen located in a middle position. Since the direction of the secondarymeasurement beam always remains the same but the reflector does not movein a straight line in the measurement beam path but on a circular path,it is evident that the movement of the reflector detected by themeasurement beam is limited. The parallel shift of the reflectedmeasurement beam relative to the incident measurement beam which iscaused by eccentric incidence of the measurement beam onto the reflectoris compensated by beam widening optics.

In order to compute the orientation of the mirror surface of any movingmirror it is theoretically necessary to measure the spatial positions ofthree points arranged stationary relative to the mirror surface. Fordetermining the orientation of a mirror rotatable about two stationaryintersecting axes the evaluation of the position of two such points issufficient.

The method according to the invention is advantageously carried outusing an interferometer with a non-shifted return beam (single beaminterferometer) which works according to the heterodyne principle. Afrequency difference between the measurement beam and the reference beamis evaluated and therefore an arm for the reference beam path asnecessary in the Michelson interferometer is not needed. A part of thelaser beam is branched off as a reference beam and is led directly tothe interfermometer receiver, the remaining light of the laser beam isfrequency-shifted by way of an acousto-optical modulator and serves asmeasurement beam, i.e. runs through the measurement beam path to themeasurement object, is reflected on this and is then led to theinterferometer receiver. The interferometer detects the interference,i.e. a signal at a frequency which arises with the superposition of themeasurement beam with the reference beam and which corresponds to themodulation frequency plus/minus the Doppler frequency. Subsequentelectronics compares this signal with respect to phase and frequency tothe original modulation frequency and produces per traversed halfwavelength, according to the direction, a positive or negative countimpulse.

The secondary measurement beams are advantageously branched out of thebeam path of the primary measurement system, after the mentionedmodulator so that only one laser source and only one modulator need beprovided.

According to the invention, the arrangement of the target trackingmirror for a laser tracking system comprises means for changing theorientation of the mirror from a predetermined zero orientation and atleast two retroreflectors co-moving with the mirror. Furthermore, itcomprises means for deflecting measurement and reference beams of atleast two secondary measurement systems out of the beam path of thelaser tracking system, means for directing in each case one secondarymeasurement beam onto one of the retroreflectors connected to the targettracking mirror, means for interferometric analysis of the secondarymeasurement beams reflected by the retroreflectors for detecting pathlength changes in the beam paths as well as means for computing thespatial orientation of the target tracking mirror from the measured pathlength changes.

The target tracking mirror is for example in a known manner mounted in acardanic suspension and is rotatable about two axes orthoganal to oneanother, for which suitable drives are provided.

The method according to the invention and the device according to theinvention are described in detail by way of the following Figs. wherein:

FIG. 1 shows the principle of determining the spatial orientation of atarget tracking mirror using an exemplary embodiment of the methodaccording to the invention;

FIGS. 2, 3 and 4 show for an exemplary embodiment of the mirrorarrangement according to the invention, in various views, targettracking mirror, mirror suspension, drives for changing the mirrororientation and retroreflectors;

FIGS. 5 and 6 show two schemas of the mirror arrangement according toFIGS. 2 to 4 illustrating the optical arrangement for carrying out themethod according to the invention (FIG. 5: block schema, FIG. 6:three-dimensional schema of the beam paths);

FIG. 7 shows a target tracking mirror, mirror suspension andretroreflectors of a second advantageous embodiment of the mirrorarrangement according to the invention.

FIG. 1 illustrates on an exemplary embodiment the principle of theinvention serving for determining the spatial orientation of a targettracking mirror. The Figure shows a target tracking mirror 1 rotatableabout an axis A with a mirror surface perpendicular to the paper plane.Parallel to the mirror surface and perpendicular to the rotational axisA there runs a connection rod 2 which is rotatable with the mirror aboutthe rotational axis A and on whose ends there is mounted in each caseone retroreflector 3.1 and 3.2. The target tracking mirror 1, theconnection rod 2 and the retroreflectors 3.1, and 3.2 are shown in tworotational positions, wherein the reference numerals of the parts arecharacterized in the one rotational position with an apostrophe.

A secondary measurement beam 4.1 (in front of the reflector brought to adiameter corresponding essentially to the reflector opening by way ofbeam widening optics 5.1) is directed tangentially to the circular arcdescribed by the reflector 3.1 on rotation of the target tracking mirror1 and advantageously such that in a middle reflector position itcentrically impinges the reflector. The measurement beam 4 is reflectedin the retroreflector 3.1 and in a manner yet to be shown together witha suitable secondary reference beam is led to an interferometerallocated to the retroreflector 3.1.

On pivoting mirror 1 and reflectors 3.1 and 3.2 the path length of thesecondary measurement beam 4.1 changes, wherein path length changesbetween a predetermined zero position (e.g. defined by mechanicalabutment or optically) and a momentary position are interferometricallydetected and measured. The orientation of the mirror 1 (characterized bythe rotation angle α) is unambiguously determined by the path lengthchange of the secondary measurement beam 4.1 which is effected by adisplacement of the retroreflector 3.1 from its zero position (for thezero orientation of the mirror) into another momentary position.

A secondary measurement system directed in an analogous manner onto thesecond retroreflector 3.2 (not shown in FIG. 1) measures an equallylarge path length change with a reversed polarity.

The measurement range of the arrangement shown in FIG. 1 (angle rangebetween two mirror orientations with maximal pivoting still detectableby the measurement beam 4.1) is dependent on the size of the reflector,on its distance from the rotational axis A and on the optical means withwhich the secondary measurement beam is produced and analyzed. Thefurther the reflector is distanced from the rotational axis A the largerbecomes the eccentricity of the beam incidence with the same rotation ofthe mirror, that is to say the measurement range becomes smaller. On theother hand with an increasing distance between the retroreflector andthe rotational axis the measurement resolution becomes larger. It hasbeen shown that for usual applications using triple prism reflectors ina ball with a diameter of approx. 12 mm a sufficient measurementresolution is achieved with a distance between the reflector 3.1 and therotational axis A in the region of approx. 100 mm. In such anarrangement there results a maximum rotation of the mirror 1 out of itsmiddle orientation by an angle α_(max) of ±15°, which for the primarymeasurement beam of the laser tracking system deflected by the targettracking mirror means a maximum pivoting range of approx. ±30°(effective measurement range of the laser tracking system).

FIG. 1 shows the target tracking mirror 1 and the reflectors in theirpositions which are as far as possible from the middle position(rotational angle equal to ±15°). The measurement range of 60° for thewhole arrangement is indicated with interrupted lines.

For a mirror arrangement with a pivoting range limited in the abovementioned manner, it is possible to use more simple and smaller motorsfor the mirror pivoting than this is common in the state of the art.Therefore, the mirror arrangement can be designed very compact. Inparticular so-called limited angle torque motors functioning accordingto the galvanometer system are suitable. In order to further reduce themoment of inertia of the parts moved with the mirror it is also possibleto arrange suitable motors not movable with the target tracking mirror,but in a stationary way and to couple them with suitable forcetransmission means (e.g. cable pulls) to the parts of the mirrorarrangement to be moved.

FIGS. 2, 3 and 4 show three different views of the target trackingmirror 1, a mirror suspension with drives for changing the orientationof the mirror and retroreflectors for determining the mirror orientationof an advantageous embodiment of the mirror arrangement according to theinvention.

The mirror suspension is a cardanic suspension in which the mirror isrotatable about an “inner” axis A and independently of this about an“outer” axis B. The two axes A and B are perpendicular to one another,intersect and run parallel to the mirror surface of the target trackingmirror 1. FIG. 2 shows the arrangement with a viewing angleperpendicular to the mirror surface (mirror with middle orientation),FIG. 3 with a viewing direction perpendicular to the inner axis A(mirror in unbroken lines having a middle orientation and dot-dashed ina position pivoted about the axis B) and in FIG. 4 with the viewingdirection perpendicular to the outer axis B (mirror in unbroken lineshaving a middle orientation and dot-dashed in a position pivoted aboutthe axis A).

The mirror arrangement comprises three retroreflectors 3.1 to 3.3,wherein the reflectors 3.1 and 3.2 are arranged as shown in FIG. 1 atthe ends of a connection rod 2 rotatable with the mirror 1 about theinner axis A and reflector 3.3 is arranged on an extension of the inneraxis A.

FIGS. 2 to 4 likewise show schematically drives 6.1 and 6.2 engaging theconnection rod 2 for rotation of the mirror about the inner axis A. Thedrives are for example stationarily mounted motors which for example viapull cables are interactively connected to the parts to be moved. In thesame way, threaded rods may be provided as force transmission means,wherein then the motors are to be assembled with a pivotable axis and arotationally secured stator.

All three retroreflectors 3.1 to 3.3 advantageously form the corners onan essentially equilateral triangle in whose center the intersectionpoint of the two axes A and B and the center of the target trackingmirror 1 are arranged. This arrangement essentially yields the samesensitivity differences for measurement of the rotation about the twoaxes A and B.

Secondary measurement beams 4.1 to 4.3 are directed onto eachretroreflector 3.1 to 3.3 advantageously parallel to one another andperpendicular to the mirror surface with a middle orientation of thetarget tracking mirror 1.

A determination of the mirror orientation with the help of threeretroreflectors 3.1, 3.2, 3.3 moved with the mirror and three secondarymeasurement systems as illustrated in FIGS. 2 to 4 is independent of themounting play of the suspension which leads to a very high accuracy.

FIGS. 5 and 6 show in particular the optical elements and beam paths ofa laser tracking system in which the orientation of the target trackingmirror 1 of a mirror arrangement according to FIGS. 2 to 4 is determinedwith the help of the method according to the invention. FIG. 5 is a sortof block schema and also shows the mirror arrangement (view as in FIG.4), the target retroreflector 22 and parts of the control system fortracking the primary measurement beam 4. FIG. 6 is a three-dimensionalrepresentation of the beam paths of an optical arrangement whichdeviates slightly from the arrangement according to FIG. 5 and whichserves the same purpose. The elements already described in combinationwith the FIGS. 1 to 4 are denominated with the same reference numerals.

The primary and secondary measurement systems shown in the FIGS. 5 and 6function according to the heterodyne method described briefly furtherabove. The optical elements and their functions in the whole system aredescribed in the following sections, in particular relating to FIG. 5.

A laser 12 of a laser tracking system emits a laser beam 13 with a verylarge coherence length. From this laser beam 13, a primary referencebeam 15 is decoupled by a beam splitter 14 and then deflected by beamsplitter 16 to be directed directly to an interferometer receiver 17.The portion of the laser beam 13 passing through beam splitter 14 runsthrough an acousto-optic modulator 18 and leaves this as afrequency-shifted, primary measuring beam 4. This beam runs through abeam splitter 20 and beam widening optics 5 and is then deflected by abeam splitter 21 towards the target tracking mirror 1 and from this isdirected onto the target retroreflector 22 (e.g. triple mirror, tripleprism or cat eye). In the retroreflector 22 the primary measurement beam4 is folded in itself and via the target tracking mirror 1 falls back tothe beam splitter 21. A portion of the primary measurement beam 4reflected in the retroreflector 22 passes the beam splitter 21 and fallsonto a position-sensitive detector 23. This detector determines thetarget error of the primary measurement beam and generates correspondingsignals which are used as a control variable for re-adjusting the targettracking mirror 1 by rotation. The target tracking mirror 1 iscontinuously rotated such that the tracking measurement beam 4 deflectedto the target retroreflector 22 does not lose contact with the targetretroreflector 22, but tracks it constantly.

The portion of the reflected primary measurement beam 4 which isreflected by beam splitter 21 runs through beam widening optics 5 in thereverse direction. Then it is deflected by the beam splitter 20 and viaa deflecting mirror 24 is led through the beam splitter 16 to theinterferometer receiver 17. The interferometer receiver 17 detectsinterference from the superposition of the primary reference beam 15 andthe primary measurement beam 4. From the readings the distance of thetarget retroreflector 22 from a previously defined zero position isdetermined.

The primary measurement system 31 (in FIG. 5 boxed in with a dot-dashedline) with the components of the laser 12, acousto-optic modulator 18,beam widening optics 5, beam splitter 21, target retroreflector 22,position sensitive detector 23 and interferometer receiver 17 as well asthe further beam splitters 14, 16, 20 and 24 required for beamdeflection is a constructional unit being per se known and in which thetarget tracking mirror 1 is applied in a mirror arrangement according tothe invention.

The mirror arrangement comprises three retroreflectors 3.1, 3.2, 3.3 andfor the interferometric measurement of position changes of thesereflectors, three secondary measurement systems which in each casefunction with a secondary measurement beam 4.1, 4.2, 4.3 and a secondaryreference beam 15.1, 15.2, 15.3. The secondary measurement and referencebeams are decoupled from the primary measurement system.

For this purpose, first a reference beam 15.1 for the secondarymeasurement system allocated to the retroreflector 3.1 is branched offthe laser beam 13 with a beam splitter 25.1. This reference beam 15.1 isdirected via a beam splitter 26.1 to an interferometer receiver 27.1allocated to the retroreflector 3.1.

Out of the modulated primary measurement beam 4 a secondary measurementbeam 4.1 allocated to the retroreflector 3.1 is branched off by a beamsplitter 28.1. This measurement beam 4.1 is deflected via a beamsplitter 29.1 to beam widening optics 5.1 and then impinges theretroreflector 3.1. Thus the secondary measurement beam 4.1 has reachedthe end of its measurement path, specifically the retroreflector 3.1itself. From here the measurement beam 4.1 runs through beam wideningoptics 5.1 in the reverse direction and then through the beam splitter29.1 and the beam splitter 26.1. Then it is, just as the reference beam15.1, led to the interferometer receiver 27.1. This interferometerreceiver 27.1 detects the interference from superposition of thesecondary reference beam 15.1 and of the secondary measurement beam 4.1.From the readings the migration of the retroreflector 3.1 from apreviously defined zero position is determined.

The same applies in an analogous manner to the other two further,secondary measurement systems which are allocated to the retroreflectors3.2 and 3.3.

FIG. 7 shows a further embodiment of the mirror arrangement according tothe invention, which corresponds to the arrangement according to FIGS. 2to 4, however, without the third reflector 3.3 arranged on the innerrotational axis A and without a third secondary system. With the use ofonly two retroreflectors 3.1 and 3.2 and secondary measurement systemsallocated to them, the central rotational point of the cardanicsuspension is used as the third point for determining the orientation ofthe mirror surface. Although this point has an essentially stationaryposition, it is however dependent on the bearing play of the suspensionso that the evaluation of the orientation of the target tracking mirrorof the mirror arrangement according to FIG. 7 in contrast to thearrangement according to FIGS. 2 to 4 is dependent on the bearing play.

A rotation of the target tracking mirror of the mirror arrangementaccording to FIG. 7 about the inner axis A results in oppositely equalpath length changes for the two secondary measurement systems allocatedto the reflectors 3.1 and 3.2, a rotation about the outer axis B resultsin equal path length changes. From this relation together with the knownposition of the axes intersection point relative to the mirror surfacethe orientation of the mirror can be computed.

What is claimed is:
 1. A method for determining the spatial orientationof a movable target tracking mirror of a laser tracking system in whichthe direction of a primary measurement beam is changed by changing thespatial orientation of the target tracking mirror, direction changes ofthe primary measurement beam are detected by determining the spatialorientation of the target tracking mirror, and path length changes inthe beam path of the primary measurement beam are detected withinterferometric methods; characterized in that from the primarymeasurement beam at least two secondary measurement beams and secondaryreference beams allocated to the secondary measurement beams arebranched off, that the secondary measurement beams are directed in anunchangeable direction towards in each case a retroreflector movablewith the target tracking mirror, that changes of the path length of thesecondary measurement beams on movement of the target tracking mirrorout of a predetermined zero orientation into an orientation to bedetermined are interferometrically detected, and that theinterferometrically detected path length changes are used for computingthe orientation of the target tracking mirror.
 2. A method according toclaim 1, characterized in that the target tracking mirror is rotatableabout at least one rotational axis and that the secondary measurementbeams are directed tangentially on circular arcs on which theretroreflectors move on rotation of the target tracking mirror.
 3. Amethod according to claim 1, characterized in that the target trackingmirror is rotatable about two rotational axes intersectingperpendicularly and arranged in parallel to the mirror surface, that theretroreflectors are arranged in a plane parallel to the mirror surfaceof the target tracking mirror and that the secondary measurement beamsare parallel to one another and when the target tracking mirror hasmiddle orientation, are directed perpendicularly onto the mirrorsurface.
 4. A method according to claim 1, characterized in that threeretroreflectors are arranged in a manner such that their positionsdetermine the spatial orientation of the mirror surface of the targettracking mirror in an unambiguous manner.
 5. A method according to claim3, characterized in that two retroreflectors are arranged in a mannersuch that they together with the rotational axes unambiguously determinethe spatial orientation of the mirror surface of the target trackingmirror.
 6. A method according to claim 1, characterized in that pathlength changes of the beam paths of the primary and of the secondarymeasurement beams are interferometrically measured according to theheterodyne method.
 7. A method according to claim 6, characterized inthat a laser beam is modulated in a modulator into the primarymeasurement beam, that in front of the modulator one primary and atleast two secondary reference beams are deflected out of the laser beamand that after the modulator the secondary measurement beams aredeflected out of the primary measurement beam.
 8. A method according toclaim 2, characterized in that the target tracking mirror is rotatableabout two rotational axes intersecting perpendicularly and arranged inparallel to the mirror surface, that the retroreflectors are arranged ina plane parallel to the mirror surface of the target tracking mirror andthat the secondary measurement beams are parallel to one another andwhen the target tracking mirror has middle orientation, are directedperpendicularly onto the mirror surface.
 9. A method according to claim2, characterized in that three retroreflectors are arranged in a mannersuch that their positions determine the spatial orientation of themirror surface of the target tracking mirror in an unambiguous manner.10. A method according to claim 8, characterized in that tworetroreflectors are arranged in a manner such that they together withthe rotational axes unambiguously determine the spatial orientation ofthe mirror surface of the target tracking mirror.
 11. A laser trackingsystem comprising: a target tracking mirror, means for changing thespatial orientation of the target tracking mirror, means for producing aprimary measurement beam directed onto the target tracking mirror with adirection which is changeable by way of changing the spatial orientationof the target tracking mirror, means for interferometric detection ofpath length changes in the beam path of the primary measurement beam,and means for detecting the spatial orientation of the target trackingmirror for determining direction changes of the primary measurementbeam, the means for determining the spatial orientation of the targettracking mirror comprising: at least two retroreflectors which areconnected to the target tracking mirror in a manner such that theirposition is changed when the orientation of the target tracking mirroris changed, means for deflecting, out of the primary measurement beam,at least two secondary measurement beams allocated in each case to oneof said retroreflectors and secondary reference beams allocated to thesecondary measurement beams, means for directing the secondarymeasurement beams onto in each case one of said retroreflectors, meansfor interferometrically analyzing secondary measurement and referencebeams allocated to each other for detecting path length changes in thebeam paths of the secondary measurement beams, and means for computingthe spatial orientation of the target tracking mirror from the detectedpath length changes in the beam paths of the secondary measurementbeams.
 12. A mirror arrangement according to claim 11, characterized inthat the means for detecting path length changes in the beam path of theprimary and of the secondary measurement beams are interferometermeasurement systems functioning according to the heterodyne method, thatmeans for deflecting a primary reference beam out of a laser beam and amodulator for modulating the laser beam into the primary measurementbeam are provided and that in front of the modulator there are providedmeans for deflecting out the secondary reference beams, which means fordeflecting out the secondary measurement beams are arranged after themodulator.
 13. A mirror arrangement according to claim 11, characterizedin that the target tracking mirror is rotatably arranged about at leastone rotational axis and that the secondary measurement beams aredirected tangentially to circular arcs described by the retroreflectorson rotation of the target tracking mirror.
 14. A mirror arrangementaccording to claim 13, characterized in that the target tracking mirroris mounted in a cardanic suspension in which it is rotatable about aninner axis and an outer axis, said axes intersecting perpendicularly andbeing arranged parallel to the mirror surface of the target trackingmirror.
 15. A mirror arrangement according to claim 14, characterized inthat the secondary measurement beams are directed parallel to oneanother and perpendicular to the mirror surface of the target trackingmirror when having a middle orientation.
 16. A mirror arrangementaccording to claim 14, characterized in that two retroreflectors arearranged on a connection rod rotatable with the target tracking mirrorabout the inner axis.
 17. A mirror arrangement according to claim 16,characterized in that a third retroreflector is arranged on the inneraxis.
 18. A mirror arrangement according to claim 17, characterized inthat the three retroreflectors form an equilateral triangle which isarranged parallel to the mirror surface of the target tracking mirrorand whose middle point is the projection of the axes intersection point.19. A mirror arrangement according to claim 13, characterized in thatfor rotating the target tracking mirror there are provided stationarymotors and force transmission means.
 20. A mirror arrangement accordingto claim 19, characterized in that the force transmission means are pullcables or threaded rods.